text:
ATC interruptions, when is it okay/overlooked?
Having recently completed my first solo flight, the tower gave me my taxi instructions, and then congratulated me, as my instructor was hanging out in the tower with the controllers at the time.
It's relatively busy airspace, so there were quite a few pilots on frequency, and multiple landing at the same time. However, before I could repeat/confirm my taxi instructions, a half dozen other pilots got on frequency congratulating me as the "new-blood". Seeing as this took about 30 seconds, it delayed my taxi readback, and potentially delayed other pilots approaching.
Controllers were fine with it, so it seemed, but I would assume that in certain situations this could be considered abuse of airport communications. Are there any guidelines for when the tower has to / should report such incidents, or is it merely up to the towers discretion?
It didn't prove any large issue in my case, other than a delayed taxi, but I'm wondering where the "line" would be drawn.

Related, and congratulations on your solo!

Pondlife - That's a very good read, and thanks!

Humans are imperfect and like to have fun, so a bit of levity is fine. Take it in stride. Oh, and congratulations on your solo!

GbD - Thank you, I didn't mind the chatter at all from the pilots, however I recognize that it could potentially cause issues - for example, the taxiback instructions only had about 3 turns this time. However, if the taxiback instructions were more complicated, like I had last night (7 taxiway turns, two runway crossings, and a taxi via a closed runway all given to me in one call!), I would easily have forgotten the instructions by the rime I was able to read back. I'm more interested in when the line should be drawn, and whether ATC has a duty to report if it crosses that line.

You should be writing down your clearances on your kneeboard. Most pilots develop a short hand for this, don't rely on your memory.

Lucky you learning at a friendly airfield. All too often the "pros" can be a bunch of elitist a-holes. I always found american ATC/pilots to be a lot more friendly/relaxed than im used to in the UK.

Ron Beyer - At this airport, an unusually busy class D (one of the worlds busiest), they often give taxi instructions before you are clear of the runway. There's no opportunity for writing it on the kneeboard.

Were you actually on the runway waiting for the frequency to quiet down, or had you vacated and were waiting on a taxiway?

I was on the runway when he gave the instructions, but but by the time I was able to readback, I had been clear of the runway and holding for about 20 seconds.

@WillSimon you did good. Exit and hold clear of the runway if you didn't hear the full clearance. Then bask in the glory of your achievement and congratulatory calls for a few seconds, then when you have a chance, ask tower to say again.

At my home field, after landing, I have to cross a runway to taxy back to park.  If I'm not automatically given clearance, or if I can't read that back because of radio chatter (not uncommon), I have to stop and wait, otherwise it would be a runway incursion!  Everyone understands that

Since you have not stated any jurisdiction in your question, I am basing this answer on the EU, which is what I am familiar with. Rules in many other parts of the world are going to be similar if not nearly identical.
Regulation EU 376/2014 lays down rules on the reporting, analysis and follow-up of occurrences in civil aviation. On the list of Mandatory Reporting you will find:

(v)   communication-related occurrences;

which I think, among the possible categories, would be the best fit for the kind of event you describe. However, it is not detailed further what is encompassed by this category, so whether something like what you describe falls under mandatory reporting, I don't know for sure. In theory, rules were violated (unrelated chatter on the radio is, quite simply, not allowed). In practise, it doesn't sound like any harm was done.
In any case, there is also Voluntary Reporting, which is designed to capture

(a)  details of occurrences that may not be captured by the mandatory reporting system;
(b)  other safety-related information which is perceived by the reporter as an actual or potential hazard to aviation safety.

The rules concerning reporting can be quite difficult to comprehend; my general advice (and what I have learned at school) is: if in doubt, file a report. If you feel the safe environment was in any way compromised, you should report it. Remember, the aim of a report is not to appoint blame, but rather to improve safety. I know from experience that it might be difficult to report something done by someone you know, since it might feel like you are pointing fingers or ratting someone out, but the truth is that the reason aviation is so safe, is because the people who work with aviation takes safety seriously, and reports violations.
As for whether the controller is going to report your situation, I doubt it - they is the one who initiated in the first place after all. But again, if you feel safety was violated, please write a report. Your writing today may save lives tomorrow.

I was more focused on US side of things, but it seems like there is a lot of crossover between US and EU air laws.

ATC communication priority on "Tower" Frequency:
If you were flying in the US:

ATC
Aircraft exceeding 31.44m in length (103ft, 2inch)
All of the smaller aircraft
On-ground vehicles

In all of the EU Airports:

ATC
Aircraft (No matter what size or load they are carrying)
All other On-Ground Vehicles

In your Case, If you were flying in Europe You should have been fine. There is no such rule about aircraft priorities in the EU airspace.

The question was: "Are there any guidelines for when the tower has to / should report such incidents, or is it merely up to the towers discretion?" The information you gave here doesn't seem to answer the question at all. Are you saying that in the United States, the reporting requirements depend on the size of the aircraft involved?


text:
What are the biggest challenges for GA airports?
I'm doing a little research into General Aviation airports and wasn't able to find any airport management forums or the like to pose this question, so I'm hopeful someone here will have some helpful insights.  (Note: I did find a community associated with AAAE, but it didn't seem to have much of a community in it)
So I'm curious to hear from GA pilots, Airport Managers, and others on what the biggest daily challenges for GA Airports are?
I'm not talking about macro issues - federal/state/local funding, scarcity of land, governmental entities run wild (see Santa Monica Airport), etc - I'm interested in the daily operational issues of the local 5000' paved GA airport.
On the outside, these airports and FBOs see to be fairly straight forward enterprises, whether run by a municipality or privately operated.  But I imagine it's not that simple.
Note: I am a private pilot and aircraft owner, primarily looking to expand my knowledge of the business of aviation - particularly GA.  This question is intentionally broad, but I imagine will yield some fairly insightful specific answers.
Edit:
I'm primarily concerned with answers pertaining to the overall business success of an airport or FBO. I understand that this might yield answers with a number of variables, but should overall have the common theme of reasons for business success or failure of the operator which is a key component of the continued operation of the airport facility itself.

Welcome! This kind of question doesn't really fit well in this Q&A site. There is no single answer, and this is strongly a matter of opinion. Maybe you can rewrite the question to fix the two problems.

Please note, on hold is not closed. If you would take the time to ask a more specific, narrower question, one that is less opinion and more factual, it will end up being reopened, I am sure. This is a topic I know I'd like to know more about, but yes, as it is it is way too broad.

I think a problem here is that "general aviation airport" covers an extremely wide range, from places like dirt/grass strips in the Idaho back country (which might see one plane a week) to fly-in communities to places like Reid-Hillview in San Jose.  One answer won't fit all.

One set of challenges that comes to mind are the types of things you find at an airport that is on that cusp between having an operating control tower and not.  I fly out of KJYO, Leesburg, VA and it very much fits into that catagory.  It frankly just gets too busy sometimes to support operations managed only by pilots coordinating on CTAF.  Examples of this include:

Wind conditions causing a conflict over whether Runway 35 or 17 should be in use.
Pilots doing straight in instrument approaches, while VFR pilots do pattern practice.
Conflicting planes on the single taxiway.

Indeed, I think the FAA understands this, since they are using it as the first US test site for a remote control tower.  The several times I've flown when this was in operation where a breath of fresh air!

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text:
Is cabin air freshened when a plane is taxiing or on the ground?
I know that commercial aircraft continuously freshen the cabin air when aloft by compressing and cooling the air.
Is that true when the aircraft is on the ground, at the gate loading or taxiing?

Related: 1: What is this tube connected to a 757 for?, 2: What is this device they bring on board from the jet bridge when we park?, 3: What is the APU actually used for?

The aircraft when on ground and starts to board passengers is switched to ground power or auxiliary power which runs the air conditioning system. When disconnected, the engines supplies the power and bleed air for the cabin pressurization.
"On the ground, the airplane is unpressurized and the outflow valve is wide open. During preflight, the pilot sets the cruise altitude on a cabin pressure controller. As soon as the weight is off the main wheels at takeoff, the outflow valve begins to close and the cabin starts to pressurize."
Source -
https://www.airspacemag.com/flight-today/how-things-work-cabin-pressure-2870604/

The air conditioning system requires compressed air to operate (called bleed air),  the ground power unit alone cannot provide a/c

when on ground the pressurization does not work only temperature control. these two sub-system are part of enviornment control system in aircraft. Once the pilot sets the pessurization on only after the engines are started and the plane usually is not heavily pressurised upto 10k feet...

with packs off there is no new air entering the cabin and thus temperature control is not possible (except perhaps the 787 which has no engine bleed and is all electrical allegedly).

If the engines are off, ie. durring boarding or while waiting at the gate, the APU can supply the necessary bleed air and electricity to operate the PACKs that supply air conditioning to the cabin. It is up to the crew to decide wether to use it or not, depending on ambient conditions: on a nice 21deg sunny day vs. a cold and snowing -4 deg.
Now that APU running in the tail, for each aircraft parked on the stand, sometimes for dozens of minutes can add up to quite a lot of noise on th apron, so most airports will have regulations as to how much time you can use the APU. Some will even cordially (for a fee) provide air conditioning through a specially mounted tube that connectes to the airplane systems, allowing cool air to enter the cabin while the APU is switched off

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text:
Why is the AWACS' rotodome located at the back and over the aircraft?
Browsing this website, I found this question dealing with torque due to the huge piece in motion, and its answers also speak of drag.
The rotodome is located above the aircraft and at the aft of the aircraft. For me it would have been logical to put this huge mass as closely as possible to the CG for stability reasons. For this aircraft, the CG is located close to the main gear, a little in front of them. When I look at an AWACS, I can see that the rotodome is located well behind the main gear.
AWACS image courtesy of airliners.net
Moreover, it seems logical to put the rotodome under the aircraft. Its mission is to observe airspace under the aircraft to detect hostiles flying low enough to avoid detection by ground radar. By mounting the rotodome above the fuselage, the aircraft's wings may disturb the radar mounted above them.
This position is also shared with other airborne radar such as the A-50 or the E-2C.

There simply isn't room to have it under the aircraft unless it retracts into the hull for landing. Look at the photo on the question you linked: the rotodome would be 10 feet underground on landing.

Another reason: Ruder and stabilizers are located in the rear of an aircraft, as this gives a more stable flight (i.e.: An aircraft would flight straight ahead when you don't move this control surfaces). Putting them to the front makes the aircraft unstable and impossible to fly by hand. The radar setup acts like a fixed rudder/stabilizer, so you want it in the back to have a stable flight.

@sweber I take it you've never seen a canard configuration? Aerodynamic instability has nothing to do with the placement of the control surfaces. Even the wright brothers had their elevator/stabilizer out in front.

The radome may be large, but it contains mostly empty space. Roughly, only the space underneath the light-gray band is occupied. All the equipment inside the fuselage is far heavier than the radome.

The radar dome has a diameter of 9.1  m , is 1.8 m thick  in the center and is fixed 3.4  m above the fuselage by two supports  . It is tilted 2 ° forward to reduce aerodynamic drag during takeoffs and when the aircraft is flying at maximum range spee.        https://fr.m.wikipedia.org/wiki/Boeing_E-3_Sentry

It's mounted dorsally in large part because it's simply too large to fit beneath the fuselage.
The rotodome has to be that large because inside is actually a planar array. The radar itself isn't round, nor aerodynamic, so it's wrapped in a smooth fairing that presents the same profile regardless of which direction the array is facing.

Caption: The Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome. (Credit: Scott Carson. Twitter.)
Some airborne radars do not rotate and can indeed be mounted below the fuselage, like the E-8 JSTARS' APY-7 (primarily a ground surveillance radar):

(Credit: USAF. Link.)
Pave Mover, an APY-7 prototype:

(Credit: Daderot. Link.)
The cancelled E-10 (planned E-3 Sentry and E-8 JSTARS successor) also had a similar under-nose longitudinal array.

(Credit: Defense Industry Daily. Link.)
However, most larger, non-rotating arrays are mounted dorsally and longitudinally, like the Boeing 737 AEW&C:

(Credit: Seattle Aviator. Link.)

[Question:] Moreover, it seems logical to put the rotodome under the aircraft. Its
  mission is to observe airspace under the aircraft to detect hostiles
  flying low enough to avoid detection by ground radar.

Not exclusively. Low-altitude penetrators aren't the only targets. AWACS is also tasked with finding medium to high altitude aircraft, both watching for enemies (eg firing "AWCAS killers") and keeping track of friendlies.

[Question:] By mounting the
  rotodome above the fuselage, the aircraft's wings may disturb the
  radar mounted above them.

You don't need a lot of depression to observe things at medium to long range. At 30,000 ft, the horizon sits just a few degree below level, and even at 30 miles (which is very close) a low-altitude target is just -10 degrees. There's also a fair bit of clearance between the elevated dome and the fuselage/wings, which reduces how much of the ground is masked (roughly -20 degrees from radar to wingtip). Together, there's probably fairly little that an E-3 wouldn't be able to see. You'd have to be practically underneath it at close range. And then you'd have to deal with the fighter escorts...

Not all AEW&C control aircraft have a huge radome on top of the fuselage. For example, the Chilean EB-707 'Condor' has the radars mounted on the sides and front.

"EB-707 Condor, Chilean Air Force (FACh) v2" by Hippocamelus - Own work. Licensed under CC BY-SA 3.0 via Commons.
There are multiple considerations in deciding the location of the radome/rotodome in an AWACS aircraft.

The AEW platform must be able to fly at high altitude such that the radio horizon remains beyond the range expected for a target flying at low altitude; at the high altitudes of operation, the detection of objects at the expected detection range near ground depends on the target discrimination software, rather than putting radar on the belly as the line of sight of the radar would be the same whether it is above or below fuselage. Also, it will be extremely difficult to position the huge rotodome (~30' dia. in E3) under the fuselage.
The range (which determines the power and coverage requirements of the radar) and coverage (the radar needs to cover 360$^{\circ}$) means that there are three options to mount the aircraft:

on a radome on top of the fuselage (like EA-3 sentry), high enough to avoid masking effects of the wing etc. This as the best option, as the range is the same in all directions and it gives 360$^{\circ}$ with a single (rotating) radar, In the rotating Mechanically Scanned Array (MSA) radar used in the beginning, this was the only option and is used in modern AESA too as it eliminates the requirement radars at multiple locations.

Image from State-of-the-Art in Active Electronically-Steered Array Surveillance Radar: Indian Value Addition by S. Christopher

on conformal antennas on the sides and in front of the aircraft, like EB-707. This reduces drag, but requires atleast four antennae as the side antennae provide only 120$^{\circ}$ coverage each and two more antennae are required in the nose and the tail for covering the remaining portion.

Image from State-of-the-Art in Active Electronically-Steered Array Surveillance Radar: Indian Value Addition by S. Christopher

on dorsal antenna on top of the fuselage (Boeing 737 AEW&C)which gives 150$^{\circ}$ coverage on both sides; this again requires multiple radars. In Boeing 737 AEW&C, there is a radar on top for ensuring 360$^{\circ}$ coverage. Also, this is possible only for modern AESA radars.

Image from State-of-the-Art in Active Electronically-Steered Array Surveillance Radar: Indian Value Addition by S. Christopher

The rotodome itself has a stabilizing influence on the aircraft due to its orientation and location.
One of the most important factors is that mounting the radar outside the aircraft enables the use of existing aircraft with minimal modifications compared to conformal radars. As the number of AEW radar operated is pretty low, this is important as commercial aircraft can be utilized.
The radar systems, sensors, operators and consoles weigh much more than the radar itself and can be located in the front to reduce any effect in the center of gravity.

The "Condor"... such a perfect nickname for that plane.

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text:
How to use/be an airborne relay
There are some cases where a pilot may not be able to reach ATC directly due to lack of power or being below the radio horizon:

On a handheld backup radio
After a forced landing
Over open water
In high terrain

What is the proper process and phraseology to request an airborne relay via an aircraft that can hear both you and ATC?
Likewise, what is the proper process and phraseology to provide such a relay?

Not sure of proper phraseology, but was done for UPS flight 6 (https://en.wikipedia.org/wiki/UPS_Airlines_Flight_6)

From the other side of the mic, there is no set phraseology for this situation. The closest thing I can find is in Section 4, IFR, of the 7110.65:

4–4–2 Clearance Prefix
a. Prefix a clearance, information, or a request for information which will be relayed to an aircraft through a non-ATC facility by stating "A-T-C clears," "A-T-C advises," or "A-T-C requests."
b. Flight service stations and ARTCC Flight Data Units must prefix a clearance with the appropriate phrase: "ATC clears," "ATC advises," etc.

Besides that, the general "exercise best judgement" provision of 1–1–1 applies and I would use plain language. In my experience, this often sounds like:

ATC: Cherokee 345, Podunk Approach, you got time for a request?
N12345: Go ahead.
ATC: I'm trying to get ahold of a Skyhawk on this frequency, Skyhawk 54321, can you reach out and see if they can hear you?
N12345: Wilco.
N12345: Skyhawk 54321, this is Cherokee 12345, how do you hear?
[silence]
N12345: Yeah, Podunk, they can hear me.
ATC: Thank you, can you tell them that the Podunk altimeter is 29.92 and if they keep coming another ten miles I should be able to contact them?
N12345: Wilco.
N12345: Skyhawk 54321, Podunk advises that the altimeter is 29.92 and reception should be better in ten miles.

I don't believe there is standard phraseology provided for pilots in this situation either; a quick search of the AIM didn't show anything useful.
As you can see from my example, in order to be an airborne relay, the best thing is to relate what was said as close to verbatim as you can. Tossing in an "ATC advises" at the front of transmission won't hurt.
In order to request an airborne relay, I would guess it's probably best to start with request for a radio check, whether on your last assigned ATC frequency or on 121.5:

You: All stations, N54321 on 121.5, radio check.
DAL: GUUUAAAARRDDD!! Guard! Guard guard guard! YER ON GUARD!
N12345: N54321, this is N12345, I hear you loud and clear on guard.
You: Thank you, can you please let Podunk Center know that I landed in a field, everything is okay and nothing is on fire?

I don't know if anything I did was proper, but I did just this sort of thing when I was bush flying in a Cessna 180 on floats in Northern Ontario in the early 1990s, and was forced down by a rough running engine (that had a failing cylinder) on an uninhabited lake, with two customers on board (I was bringing out two lawyers(!), on a fishing trip, from a remote cabin about 5 miles away from my landing spot). In my case it was to relay a message to my base, because although I was 80 miles from civilization, things weren't serious, yet.
It was a nice sunny May morning.  It wasn't quite an emergency and my main goal was to contact my base to come pick us up, not trigger a formal official rescue operation.  The lake I landed on was part of the same outfitter's camp network, and there was a very convenient dock, from which a trail led to another lake, with a couple of the outfitter's fishing boats ready for use.
So, I sent my passengers (who seemed to be amused by the whole situation) fishing while I tried to raise passing airplanes I could hear in the distance on unicom 122.8 and on 126.7.  But no joy.  Now what to do... set off the ELT?  Then I noticed a contrail overhead and thought "I wonder..." knowing that airlines routinely monitor guard when over sparsely settled areas.
I broadcast a PAN PAN PAN call, with my registration, on guard, 121.5.  To my amazement, after a few tries a Canadian Airlines flight, the contrail above me, responded and offered help.  I was able to have them contact my base on the unicom frequency and advise my location, that all were well, and that we needed rescue due to mechanical problems, before the Cessna's meagre comm radio transmitter faded out for the airliner. I could listen to the airliner talking to base on 122.8 on the airplane's other comm and knew that help was on the way (the air service manager was suspicious it was some kind of practical joke initially).
My boss showed up with the Beaver about 5 hours later to fly us out, and I went back in the next day to help the air service's mechanic remove the engine and fly it out for overhaul.
I took the pic below while preparing a bipod from pine poles to lift the engine, using a come-along.  The mechanic is sitting in a boat we set across the floats so he could work.  The Beaver showed up late in the afternoon and we manhandled the O-470 into the Beaver by dragging it up poles into the cabin door using the come-along.  We flew the rebuilt engine back in about 2 weeks later and reinstalled it, and I flew it the rest of the season.

Great story, and a good answer highlighting that life, (and aviation) isn't scripted.  Phraseology aside, the primary purpose of communication is to... communicate.  Sometime concise plain English is the best choice.

Thanks. Funny thing was I knew ATC (Toronto Centre, which covers a large area into north eastern Ontario and probably the control tower at an airport about 100 miles away) would have heard the airline's part of the communication and I expected to get an inquiry from Transport Canada asking what happened, but never did.  As my boss arrived in the Beaver, he announced he had some fuel for me - he thought I'd landed because was low on gas.  I said "I have lots of fuel Bob. A cylinder was coming off." (hold down studs had failed).  He said "Ohhh Sh***t". Fortunately it was on 122.8.

This is an exchange summarizing what I’ve seen in videos, but I don’t have a source to prove that it is correct (if any such source even exists):

N12345: Any Aircraft, N12345, request relay to Podunk Center
ABC678: N12345, ABC678, go ahead for relay
N12345: ABC678, N12345, relay [message]
ABC678: ABC678 copies N12345 [message]
N12345: Readback correct, N12345
ABC678: Podunk, ABC678, did you copy that?
Podunk: Podunk copies N12345 [message]
ABC678: Readback correct, ABC678

Note that this assumes all three are using the same frequency, so Podunk should be able to hear ABC678 and copy their readback.  If they were on different frequencies for some reason (or if Podunk missed part of the message), the relay would have to repeat it.

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text:
How can an aircraft hover almost vertically?

(Source: https://fat.gfycat.com/)
The aircraft in this GIF (although being an RC plane) seems to be flying vertically, just meters above the ground. How is this possible? How is it possible to control it in this scenario? Would this be possible with a real aircraft?

I'd bet my house that that isn't a real aircraft

It's radio controlled, the RC on the video kinda gives it away

I did some editing magic, maybe this will salvage the question to be on topic, even though it's an RC plane, the question whether this is possible for real aircraft is valid.

@SentryRaven: That seems to be a problem to be solved by Mr Musk.

@mins Nice find! :)

@sentryraven: thank you for the edit. I feel a bit dumb for not realizing it was an RC-plane in the first place, I was simply mind blown of the maneuver.

I'm surprised that this turbojet/ducted fan aircraft can do that... I've seen propeller-driven aircraft able to do this, and the control was provided by the airstream flowing over the aircraft control surfaces due to the propeller wash. I wonder if this model has some sort of thrust vectoring going on.

hmm, yes I believe it must because as you stated, there is no airflow over the control surfaces

The aircraft in the video is a remote-controlled (rc) airplane. Why it can "hover" is simply:
The thrust generated by a big fan / rc-jet-engine is much higher than the weight of the plane.
If the force generated by the turbine is the same as the downward-force of weight of the airplane, it hovers.
By very carefully adjusting the thrust and placing the aircraft in this "nose-up" position, a skilled rc-pilot can hover the plane until the battery runs out or fuel gets low.

As example:
An aircraft that weights 10,000 kg's produces a downforce:
$$F=m\cdot g=10,000 \cdot  9.81  = 98,100N $$
To make this aircraft hover, you have to create a upward force of at least $98,100 N$. If you manage to do that and you can align the generated thrust / downforce perfectly on the Y-Axis, the aircraft will hover:
$F_{down} - F_{up} = 0N$, the two forces will cancel each other out and you don't have any movement in the Y-Axis.
However, if you have engines that create more thrust than the downforce, the following will happen:
$F_{down} - F_{up} \geq 0N$, that means the aircraft will climb vertically (On the Y-Axis).
The same principle applies to other "real" aircraft, like the Sea-Harrier:

It uses the engine to create a force facing upwards. Because the force is greater than the downward-force created by the aircrafts weight, the Harrier can land / takeoff vertically.
The hovering airctaft is controlled by a trust-vectoring fan / turbine. Controlling the plane with the "normal" surfaces is not possible, because the airflow over the controll-surfaces on the wing is to slow. Therefore, only planes with trust-vectoring capabilities COULD hover like the rc-plane in your question.

So this maneuvre is impossible with a real jet?

@bos In theorie. As long as the upward-force is greater or the same as the downward-force, you can hoover any object. Keep in mind that a pilot, to make it in a real jet happen, has to keep the balance perfectly.

You have about 3 zeroes too many in the example.

@JanHudec Thank you, eddited that.

@jklingler, you only removed them from the thrust, but you should have removed them from the mass as well.

@bos "If you impart a sufficient thrust vector, even a pig can achieve orbit."

How is it possible to control it in this scenario?  (quoting from the question) This answer does not seem to address that. Is it only possible on thrust-vectored aircraft or is some other magic involved?

@sanchises The aircraft is controlled by a trust-vectoring turbine / fan. The airflow over the control surfaces on the wing is to slow to steer the aircraft.

Perhaps you could explicitly mention that this is then only possible with aircraft that have thrust vectoring (obviously including the Harrier)

@sanchises Done!

Hovering is far easier and more common is propellor driven RC planes than jets. Very few RC jets have thrust vectoring, and even fewer have hovering capabilities.
Propellor driven aerobatic RC planes are able to hover, also due to a thrust to weight ratio > 1, however, they are controlled by the normal control surfaces rather than thrust vectoring nozzles. Hovering is only possible in models that have large control surfaces and large propellor diameters. The large surfaces are needed to attain the necessary control, and large propellor pushes more air over those control surfaces, making them more effective.
When hovering, the elevator and rudder are used to keep the nose pointing straight up, and the ailerons are used to prevent rolling due to engine torque.
The video below shows several examples of hovering, as well as other slow speed aerobatic maneuvers.

This is true for rc-aircraft with a propellor, but an rc-jet, like the one in the question, or an rc-plane that has the propellor on the tail, can't maneuver with the controll-surfaces.

These days, thrust vectoring is not that uncommon in RC "jets" or more specifically in RC planes with electric ducted fan motors, especially in scale models of military jets that have thrust vectoring.

Some fighter jets have something called thrust vectoring which moves the engines and changes the direction of the thrust, so they can hover.

Its an rc basically you have more power more thrust and the airplane body is light as paper so u can hover with thrust and control with normal control surfaces.
In real aircraft its hard.

Control is via paddles in the exhaust flow since flow over the control surfaces is of very low speed, but otherwise I agree. With aircraft like the X-31 it is even possible in real aircraft.

@PeterKämpf: For RC turbines control is typically via thrust vectoring -- moving the nozzle itself


text:
Why do aircraft need a vertical tailfin, but birds don't? (and lots of fish do?)
I don't know of any bird that has a vertical tailfin, but apparently aircraft need them for lateral stability. Why is this?
I did realize, however, that most fish have a vertical tailfin, or a vertical dorsal fin, or both. What exactly is going on there? Can't see how marine creatures would need lateral stability at low speeds in a thick liquid (compared to air).
If that second paragraph sounds off-topic, let me put it another way. What is it about the air, as opposed to water, that requires this? They are both fluids. One is thicker/more viscous than the other, which seems to imply that the thicker one would require less lateral stability. Yet it seems to be the opposite. I hope this will be taken as a valid on-topic aerodynamics question.
(P.S. I'm leaving out stuff like the B-2 Spirit because it uses active dynamic adjustments to stabilize. Its form is inherently unstable.)
IMPORTANT EDIT: I should have pointed out the large difference in speed. Birds of course are slow. So were the early Wright flyers. Modern airliners fly much faster, around Mach 0.85. I haven't yet looked up Reynold's Numbers, but it certainly could be the case that the faster ones need lateral stability while the slower one's don't.
1 more clarification: When I say "stability", I'm not exactly talking about yaw control. My impression is, just when the plane is flying perfectly straight, there are pressure fluctuations that will cause slips unless there is a large vertical surface area to balance out those fluctuations. I could be wrong, and ultimately I'm trying to find out why most planes seem to need this large vertical area.
1 more: Here's an example model that seems synonymous: Let's say our airplane achieves yaw with butterfly flaps at wingtips, which are aligned with the center of mass (the flaps and CoM have the same height) so there's no side-effect torques along another axis other than the yaw axis (z axis). Let's assume no backup yaw control is necessary. This craft is meant to cruise at Mach 0.85. Is a vertical fin necessary to achieve passive directional stability about the yaw axis? We should probably consider cross-winds too, even at takeoff/landing.

You pretty much answered your own question with the PS:  animals can continuously adjust their tail very rapidly... airplanes cannot.

@abelenky If that's true then the irony is inescapable: birds and maybe even insects "invented" active dynamic control millions of years before Boeing?

Nothing ironic about it: Engineers have long wanted totally dynamic, organic-mimicking control systems, but its only recently that sensors, actuators, and algorithms have gotten good enough.

@DrZ214 What's ironic about that? Insects "invented" flight long before the Wright brothers. Fish "invented" swimming long before we built submarines. Primates "invented" intelligence long before we got anywhere with AI.

@DavidRicherby All of those sound ironic to me. Pretty much anything that requires advanced science and billions of dollars for us to create, yet is the easiest thing for nature even a million years ago, would be ironic in the way I understand that word. To each his own.

Ask nature what you want to know. The science field is named biomimetics.

Who says that Boeing invented active control? They avoided it like the plague before warming up to the idea.

Fish typically have a vertical tail fin but marine mammals have horizontal tail fins.  This is due to the mammalian spine having a design that bends with more power in the ventral/dorsal direction while fish have a spine that bends with most power in the lateral direction; the tail fin for both is the primary means of generating thrust, unlike an aircraft.  The dorsal fin on both fish and mammals helps provide roll stability - an aircraft does this with its wings (fish and marine mamals typically have much smaller front fins so the dorsal fin assists here).

@PeterKämpf If not Boeing, then idk who first did it. When were they avoiding it? 1960's, 1970'? From what I know about computers, I would also avoid it like the plague unless it was mid-80's bare minimum.

@J... thanks for that info. So how would marine mammals do roll stability? I think the blue whale, for example doesn't have any vertical fins anywhere. Of course that's a huge beast so maybe it doesn't need much stability.

@DrZ214 Some mammals do have a dorsal fin - like the dolphin or killer whale.  The blue whale, porpoises, etc, don't, but they have much larger pectoral fins - rather like an airplane has wings, which it uses for roll control.

@J... That part makes sense but aren't there also fish/mammals that have a vertical tail and a vertical dorsal fin? I cannot think of any names but there must be so many fish like that. What are their roles (lol) in that case? BTW are you from biology.SE? Welcome to the site.

@DrZ214 I linked a relevant paper in a comment below Peter's answer - the dorsal can be used to assist with yaw control for quick turns and, in some fish, actually plays a role in propulsion (setting up constructive vortices that increase propulsive efficiency).  And no, haven't set foot in bio SE.  I'm a physicist and engineer, but I hail primarily from SO.

The vertical tail (or horizontal in whales & dolphins) isn't for stability, it's for propulsion.  For the rest, you answered your own question in your PS about active stability in the B2 - that's exactly what birds use, though with a much better implementation :-)

Not all of them need a fin:

This is the Horten IV, a flying wing glider that did not need a fin (picture source). Instead, it used spoilers at the wingtips to create yawing moments, and the swept wing helped in improving its weak directional stability. It could afford to do so because it was a glider. The second prototype of a jet-powered flying wing, the Horten IX V2, suffered a crash when one engine failed and the pilot could not end the resulting spiral dive.
Fins on airplanes
Birds have two advantages over airplanes which make a fin superfluous:

A variable geometry wing, where wing span, sweep and incidence can be controlled for each side independently.
Propulsion is integrated into the wing, instead of separate power plants which might fail independently.

An extra advantage of birds over older airplane designs lacking a computerized control system is that the bird's brain is wired to adjust the wing on each side subconsciously and continuously. While a fin provides an aircraft with weathervane stability, a bird is inherently indifferent (neither stable nor unstable) and needs continuous tweaking of its wings for directional control.
If a bird wants to control yaw, it simply adjusts its wingtips such that the drag difference between both creates the desired moment. It can do this by reducing wingspan on one side, which also creates a rolling moment for a coordinated turn. Airplanes unfortunately cannot do the same.
Airplanes need positive directional stability because humans cannot adjust the controls continuously - they need to check maps or work the radio, or maybe just want to relax for a moment. Also, multi-engined designs need a stability margin to compensate the asymmetric thrust resulting from a failed engine. With a fin, the mass of the structure behind the center of gravity now needs a compensating mass ahead of it, and the side area of this forward fuselage is de-stabilizing the aircraft in yaw, requiring yet more area at the vertical tail. This is another problem that birds do not face.

The B-52H 61-023 suffered a structural failure of the fin on January 10, 1964, but managed to limp home with almost no lateral stability left. Had it suffered an additional engine failure, it would had crashed (picture source).
Fins on fish
The dorsal fin of fish helps in their propulsion by balancing the areas above and below their vertebral column. By wagging the tail, they create a side force which creates a sideslip condition at the dorsal fin, which in turn produces a correcting side force so the body does not roll while swimming. In this it can be compared to the keel of a sailboat. The hydrodynamic forces on a keel counteract the side force from the sail in crosswind conditions. Its main function is to stabilize the fish from the yawing motion caused by their tail fins, and this stabilization in turn improves the efficiency of the motion of the tail fin.
An additional function is as a roll sensor. Being located at the maximum girth, it gives the best readings of local sideways motion due to roll. This is the main function in marine mammals where the tail fins are horizontally oriented. In fish which have an adipose fin (located aft of the main fin), this fin is most likely their roll sensor.
It is not used for roll control (except as a sensor in a feedback loop), and cannot because it is not adjustable. Roll control is achieved by the adjustable pelvic fins. It also cannot provide roll stability - this would require it to roll the fish back to its initial attitude after a disturbance. All it can do for the roll movement is to dampen it a little and prevent it when the fin is moved sideways.

Thanks for addressing both birds and fish. But I'm surprised at the answer. It seems like you're saying that airplanes don't really need vertical stabilizers for stability (from crosswind or side-slip), but rather that they need/want rudders either as the only yaw control or backup to other yaw control, and tails are just the most convenient rudder location. I was under the impression that the large surface area of the vertical stabilizer was needed to balance out some kind of random pressure fluctuation that comes from airflow over the fuselage.

@DrZ214: No, I did not say that. I said that airplanes need positive directional stability, which makes them different from birds which don't, and that is achieved by the vertical. Now you have something hanging out at the back and need to balance it with something hanging out at the front, which needs more area in the back again. Without the need for a tail there would be little fuselage ahead of the cg, which creates instability and requires a fin.

A variable geometry wing, where wing span, sweep and incidence can be controlled for each side independently. - Propulsion is integrated into the wing, instead of separate power plants which might fail independently. but suddenly, when speaking about wing terminations, birds are the example upon which airplanes should be built.

@DrZ214: The vertical stabilizer is not primarily needed for "stability" in the sense of steady-state stability. It is needed to turn. Before the Wright brothers, most gliders and model aircraft and kites were built without vertical stabilizers. And they flew well. Only, they were never intended to turn and was only flown in a straight line. Once people started tackling powered flight they discovered that for most designs turning an aircraft will cause it to crash (it will simply spiral uncontrollably into the ground)...

... of course, we know from the Horten brothers and Jack Northrop that it is possible to design planes without a vertical fin - just look at the B2. The key it turned out has been known among boat designers for a thousand years: your center of lateral resistance must be behind your center of effort. Or to translate it to airplane-speak: your center of drag must be behind your CG. The vertical fin is one way to achieve this. Split elevons is another (Northrop) and washout/twist is another (Horten)

@DrZ214: Note that the effect crashing when trying to turn I mentioned above has a name: Adverse Yaw. The Wright brothers invented the rudder to solve it and we still teach pilots coordinated turns (another Wright invention) to this day to counter adverse yaw. Planes with extreme washout have negative adverse yaw - proverse yaw. Any aircraft design that have proverse yaw don't need a vertical tail.

The dorsal fin on fish is for roll stability, primarily, not to balance thrust from the tail.  In fact, some fish don't have dorsal fins at all (or only very small or undeveloped fins), despite being powerful swimmers.  Furthermore, most cetaceans have dorsal fins and, like all marine mammals, have horizontally oriented tail fins and produce thrust in entirely orthogonal directions to fish.  They still need the dorsal fin for roll stability, though (also for more powerful yaw in sharp turns).

@J... and for what are the pelvic fins, if not for roll control? And what about seals, Belugas, or penguins, do they have a dorsal fin? All use their flippers for roll control. No, the dorsal fin counteracts the side force from the tail fin. Of course it helps in stabilizing roll, but it is more important in yaw, together with the anal fins.

@slebetman The question is about the fin, not the rudder. Flying without a rudder is possible if ailerons (and, ideally, drag rudders) are installed. A rudder merely helps to enable coordinated movement around all three axes.

@PeterKämpf Naturally each case is unique, but I won't yield the point that your statement in the answer is woefully incomplete and, in most cases, just wrong.  Strong tail swimming fish often lack an appreciable dorsal fin - it is not to counteract lateral forces from the tail.  Dolphins, on the other hand, have substantial dorsal fins and their tails don't move laterally at all.  Where dorsals are absent, usually there are strong pectoral or ventral fins for roll control.  Penguins have wings, belugas and seals have strong pectoral fins, etc.

@Federico which is exactly why early aircraft designs mimicked birds, and why they failed miserably. As mentioned in the original answer, it's only been fairly recently that we've had the computer power to make the multiple, minute corrections that birds do naturally to enable stable flight.

@PeterKämpf For additional interest, for some fish (at least Teleostei), even use the dorsal fin for propulsion - it assists in creating vortices that react constructively with those from the tail fin to increase propulsive efficiency.  This is not strictly in line with what you have suggested (lateral force stabilization), but it is closer to that type of function.  This behaviour, naturally, is not universally true of all marine animals with dorsal fins, however.  http://www.people.fas.harvard.edu/~glauder/reprints_unzipped/DruckerLauder2001b.pdf

@PeterKämpf: A rudder is just an adjustable fin. The fin serves the same purpose as the rudder: to improve spiral stability. If all you need is to travel in a straight line then a dihedral is enough to give you stability. I've built lots of free-flight planes that way without a vertical fin and they all fly fine. The problem begins when you try to trim the planes to fly in a circle. An adjustable fin (a rudder) merely allows you to get away with using a smaller fin.

@slebetman (and @PeterKampf) We need to distinguish between low-speed stuff (Wright Bros flyer, model planes) and modern high speed (Mach 0.85 for large commercial airliners). The faster one might require a vertical fin surface for stability (not yaw! not yet), and the slower one might not. I will edit the OP with that. Also Kapmf, Now you have something hanging out at the back and need to balance it with something hanging out at the front. I don't understand. What is hanging out the front, e.g., of a modern jet like a 747?

@PeterKämpf The question is about the fin, not the rudder. Yes exactly, but I understand it's easy to get confused especially as we're considering fish, birds, slow aircraft, and fast aircraft. Let me rephrase something. Let's say our airplane achieves yaw with butterfly flaps at wingtips, which are perfectly aligned with the center of mass so there's no side-effect torques along another axis. Let's assume no backup yaw control is necessary. This craft is meant to cruise at Mach 0.85. Is a vertical fin necessary to achieve passive directional stability about the yaw axis?

@DrZ214: Yes. Butterfly flaps would need to be actively moved in response to sideslip. At Mach 0.85 you need wing sweep; this alone gives reasonable directional stability if no forward fuselage is present. The fin is really to counteract the negative stability contribution of the forward fuselage (and to have margin for asymmetric thrust). Adding a rudder gives yaw control and the possibility to trim in yaw.

@J... Where did I say the dorsal fin "balances thrust"? Don't read things into my answer which are not there. I edited it for clarity and will not retract the first sentence. If the fish body would move sideways in response to the tail movement (conservation of momentum; it's not my fault if biologists don't get this), the tail would be less effective. A tall body does the same; this is why most fish are much taller than wide.

@PeterKämpf: If you have both butterfly flaps open you don't need active control response. Northrop's earlier mechanical planes had a feature to open both for "emergency stability". Granted this is a high drag configuration so if you can afford an active control system you'd prefer using it. Still, the B2 use this passive stable configurations at low speeds.

@DrZ214: Below Mach 1 the rules for tailless aircraft are the same. I'm not sure what changes at mach 1. In your scenario, if the plane is stable at take-off speed with no vertical tail then it will be stable at Mach 0.85 with no vertical tail. Indeed, for some designs like the Hortens the weakness is actually low speed flight. At high speed Hortens are very, very stable. At low speed they can benefit from a vertical tail.

@slebetman: Open butterfly flaps help only when wing sweep puts them behind the CG, a condition that was clearly ruled out ("which are perfectly aligned with the center of mass"). No, they need to be actively moved, a high drag configuration won't help (it does improve damping, though).

@PeterKämpf Oops, what i meant was, perfectly aligned with the CoM on the lateral axis. In other words, the wing span runs right thru the CoM inside the fuselage, thus the butterfly flaps have the same height as the CoM. Sorry for the inaccurate wording. They could be ahead or behind the CoM as viewed in the xy plane. Behind makes more sense to me just because I know the old rule of thumb that the CoM must be ahead of the aerodynamic center for "weathervane stability".

@DrZ214: That was my interpretation as well. Since that was what you intended my answer stands. As I said. The rule is that you simply need more lateral drag behind your CG than in front. Yes, it's like a weathervane. To the best of our current knowledge there are 3 techniques: put drag rudders (the butterfly flaps) behind the CG (Northrop solution). Put washout/twist behind the CG (Horten solution). Put something vertical behind the CG (Wright solution). Of course, nothing stops you from combining them if you still want to increase stability.

@PeterKämpf You did not suggest that the dorsal balances thurst - you're suggesting a cooperative role with the tail and I provided a research paper that is as close to your proposed role for the dorsal fin as I could find.  That said, you've several times misinterpreted my statements - I said the dorsal is for roll stability, not roll control.  These are very different.  As a roll stabilizer the fin reduces the tendency to roll by providing drag about that axis.  Roll control implies that it is actively used to adjust the roll angle of the fish - naturally this is not the case.

@J... So why did you speak of "roll stability" when you mean roll damping? Roll stability means the vehicle moves back to its old attitude after a disturbance. A static fin close to the center of gravity cannot do that, no matter what is written in some papers. All it can do for marine mammals is roll sensing and some roll damping. Please understand the terms first when criticizing things that are correct but seem strange to you. In fish, it helps in reducing the counter-movement of the body on top of the above, thereby increasing propulsive effectiveness.

@PeterKämpf The term stabilizer does not necessarily imply a direct correlation to dynamic stability (in the sense you mean).  In nautical systems it is common to use the term "stabilizer" when referring to a ship feature that provides damping.  This even extends to things like gyroscopic stabilizers, which do absolutely nothing for dynamic stability, but provide only damping against rotation about whatever axis.  I don't think my meaning was unclear.  In any case, to be clear, yes I have always meant its action to be that of roll damping, as you say.

@J... This being Aviation SE, I prefer to use the aeronautical nomenclature, where stability and damping cannot be exchanged, but have their own meaning. I was really pissed by your repeated assertion that my answer is flawed when in fact your comments were.

@PeterKämpf I'm sorry you are upset - that wasn't my intent. In any case, the action is very different from a ship's keel - by virtue of its massive weight (and placement at the bottom of the boat), a keel very much is a dynamic stabilizer in the sense you mean (that it tends to combat roll and right the ship in the water).  With the keel controlling roll, the ship's rudder provides the primary balancing force against the imbalanced thrust on the sails (yaw control).  This is very different from fish in so many ways, and different from what you have suggested in your answer.

@J You are right, the hydrostatic contribution of a keel is quite different. I guess I need to clarify the answer. However, when sailing close-hauled the keel sees quite some sideslip angle and creates considerable hydrodynamic forces. These are quite similar to the forces on a fin, and that was I was referring to. I was only thinking of dynamic forces. A keel and the rudder on boats are very much like a wing and an empennage on aircraft.

@PeterKämpf Indeed. Another difference with fish is that the tail side forces, although imbalanced on a single stroke, always alternate with regular frequency - side motion from one stroke is balanced by an opposite motion on the reverse stroke. As you say, the tall and flat profile of the fish's body provides most of the resistance to this motion - particularly in fish, which generally have softer and lighter dorsal fins. Rigid cartilaginous fins, like on dolphins, are particularly ill explained here as they generate no side forces with their tails. They do still need roll damping, however.

@J... Most mammalian (and all in case of penguins or seals) roll damping is provided by the flippers - they are larger and have a higher aspect ratio. Also, they allow active damping. The little extra from the fin is not really needed - roll damping is not its main purpose. In fish the fin helps in balancing the area above and below the vertebral column, and in this respect it helps against rolling caused by the sideways motion of the body when swimming. If you count this as helping in propulsion or aiding in stability is more a semantic than a scientific difference.

@PeterKämpf So what would you say is the role of the dorsal fin in mammals that have them?  Dolphins, Orca, etc?  Again, they do not generate any lateral forces with their tails when swimming. The generally (scientifically) accepted functions in these animals is primarily for roll damping, with an auxilliary function for thermoregulation (they can flood the dorsal fin with blood to release heat, if needed) and, in some cases, to add yaw control for quick turning.  I think if you have an alternative idea with merit it is probably worth publishing.

The vertical fins of fish and sharks may have as much or more to do with propulsion than stability.

Not addressing the question as asked, but related: Birds have "active stability control" for lateral stability. Birds don't need a rudder for coordinated turns (like most airplanes do) because they have a more-or-less bell shaped lift distribution curve. See: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023801.pdf
This lift distribution results in proverse (as opposed to adverse) yaw. With other lift distributions the increase in lift on the rising wing in roll produces additional drag, resulting in adverse yaw (there are of course other ways to eliminate this). With the bell shaped lift distribution, the increase in lift on the rising wing reduces drag, while the decrease in lift on the falling wing increases drag. See http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-106-AFRC.html for a modern unmanned aircraft demonstrating this.


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Can ramjets reach higher thermal efficiencies than gas turbines?
Since a ramjet isn't constrained by turbine inlet temperature like a gas turbine is, it seems to me that it should be able to achieve a higher thermal efficiency, given that it's operating at a speed that results in a comparable compression ratio. Is this true?

The problem is, the nozzle is less efficient than a turbine and fan combo, which is why turbofan is more efficient than turbojet and also afterburner isn't very efficient at all. Game is already lost when you are jetting hot stream directly to generate thrust, unless you are at high altitude where air is too thin to be efficiently driven by a fan.

@user3528438 You are talking about propulsive efficiency which is not the same thing as thermal efficiency.

To put it another way, exhaust temp is too high for ram jet to be very efficient.

@user3528438 I don't see why the ramjet would have a higher exhaust temperature if it runs at the same or higher pressure ratio as the gas turbine?

@PeterS The combustion product in a turbine cools by doing work on the turbine blades (work that either compresses the incoming combustion air or directly propels thrust air via the fan), so is cooler before it enters the tailpipe, even if combustion temperature is the same.  Raising combustion temperature in the ramjet makes this worse, and there isn't enough pressure differential to make up the difference with nozzle design.

@PeterS Also, you're operating with a false assumption -- ramjets do not and cannot operate with a similar pressure ratio to multi-stage compressor turbines.  Their efficiency maxes out at lower compression: https://aviation.stackexchange.com/questions/72895/about-ramjet-efficiency-in-supersonic-speed-is-there-an-optimal-speed?rq=1

The Carnot condition notwithstanding, whether a ramjet actually operates at a higher "hot end" temperature than a turbojet is questionable, as well as whether a comparable percentage of thermal energy in the exhaust stream is converted to kinetic energy.
Further, just because there's no turbine, doesn't mean there's nothing in the hot part of the engine that can melt -- combustion chamber walls are made of real material, as is the flameholder, and due to cost of exotic metals, likely aren't as heat resistant as first stage turbine blades.  Even a very simple, low-powered ramjet (see YouTube) can operate at a temperature that makes the hot section glow from heat; one capable of propelling a high speed aircraft will presumably run even hotter, requiring tradeoffs in efficiency to prevent thermal damage.

I know that rocket engines run substantially hotter than gas turbines by using regenerative and film cooling of the combustion chamber and nozzle. The number I'm finding is 3500 K for rockets compared to just about 2000 K inlet temperature for bleeding edge gas turbines. Is there reason why a ramjet couldn't run that hot as well?

Rockets use far higher mass flows of fuel and oxidizer than ramjets, which have only fuel flow to apply.  Further, if you correctly measure efficiency by "work done" vs. "fuel energy consumed" film cooling with fuel directly reduces efficiency by consuming fuel without doing additional work.  Regenerative cooling with kerosene has limits, which largely define the difference between Jet-A/JP-4 and RP-1.  Short version, if you're regen cooling with kerosene, you'd better be using RP-1, especially since a ramjet is expected to operate far longer than a rocket engine.

Planes like SR71 aren't real ramjet's but ramjet hybrid with engines to achieve desired speed.
Missiles like Talos use pure ramjet engine and achieved impressive ranges of 130 nautical miles.
At the speeds it travel's it's less a matter of temperature and more a matter of pressure; as they use same "Candle" to ignite introduced fuel as typical jet engines; temperature ranges aren't really all that much different. Thermodynamic effciency isn't really relevant for a machine that doesn't have a power conversion stage (compressor, turbine) however they can run Hotter (2,500 Degrees C) thus greater conversion of combustion fuel products reduces stoichometric hydrocarbons residuals.

A ramjet is a heat engine, so thermodynamic efficiency is very relevant. The "power conversion stage" is just clever ductwork instead of rotating machinery.

True, but the ramjet's thermal efficiency don't matter much if it's not working, it requires minimum threshold Mach 2.5 to 3 range to work properly. Second, Modern fighter jets traded turbojet's for turbofans which saw improvements in efficiency.
https://www.sciencedirect.com/science/article/abs/pii/S0094576516310207


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What medications for depression are approved by the FAA?
I understand that there may be additional medications on the approval list.


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Why do new engines have a case with a triangular tiling pattern?
Take a look at the case of new engines such as the F-135:

or the F-119:

Why do they have a case with a triangular tiling pattern? Is this a new technology? A new material?

This type of structure is called isogrid structure. Solid walls are usually very heavy and therefore engineers use more weight-efficient structures such as sandwich structures or stiffened structures with stringers and frames. A downside is the huge manufacturing cost compared to other options.
As you can see from NASA Report CR-124075 "Isogrid Design Handbook", which was written 50 years ago, it is not a new type of structure.

Damn you, you remembered the isogrid, totally escaped my stress-demented mind  I think in the case of these jet engines, the manufacturing costs are not huge if we think about the viable options that fullfill the design goals: there might be none, actually.

I think we can go further back than NASA - how about the Vickers Wellington, for example?

@MikeBrockington That was a geodetic airframe, like the airship S.L.1. I don't consider them to be the equivalent structures because isogrid structures consist of a load-carrying skin with stiffeners forming triangles, while geodetic ones are a frame structure with many rectangles and mostly a non carrying skin like fabric.

The isogrid principle appears identical to the geodesic or geodetic construction of the R100 airship, Wellington bomber and Buckminster Fuller's domes. Is there any definitive difference?

@GuyInchbald They're optically very similar, but see my comment before. The skin of the three examples you mention is mostly fabric and cannot carry loads as the isogrid skin does. From a structural point of view one is a frame structure while the other is a stiffened shell.

"The skin of the three examples ... cannot carry loads" - Not correct. A skin of doped canvas, or of plywood, or whatever else, may have less strength than metal, but in both cases it is the COMBINATION of both that complements the particular characteristics of each.

@MikeBrockington "...cannot carry loads ... as the isogrid skin does". The skin will obviously transmit some aerodynamic loads to the primary structure, but it is not part of it.

@Gypaets Thank you, that makes sense.

This is simply a way to achieve required rigidity/strength and vibration dampening with less material than in a solid, even thickness  design.
Varying the weave pattern gives a relatively easy and a very efficient way to model and produce varying properties to different sections, without having to change the "base" structure thickness.
Successfully utilizing it does require advanced computing and manufacturing abilities, therefore it was not being used commonly in more ancient times.

I wouldn't say that it was at all uncommon in older times (depending on your definition of "more ancient".  Consider the common I-beam, for example, or structures build using tubing rather than solid rods, e.g. the tube and fabric structure of older airplanes.

You need to get fairly ancient: the Schütte-Lanz SL-1 utilized it in 1909, and Vickers-Armstrong used geodetic airframes in the 1930s.  The calculations are complicated, though, which is why use of the technique only became common recently.

The structures you mention are somewhat different. The engine casings or shells in the pictures are are inseparable uniform solids, where as in the case of the airships and the V-A the skin and the grid were separate. Maybe glued or riveted together, dunno, but still separate "entities".

It might seem that the wimpy little ribs would not add much stiffness. What is invisible in the pictures is that the outside of each triangular facet is flat, but the inside is curved to form a continuous smooth circular cylinder. Therefore the middle of each facet is very thin, but the ribs surrounding the triangle prevent any impact damage from an internal failure in the engine from spreading across the whole casing. (In fact, I remember one "accident" where the company making a prototype casing actually cut right through some of the thinnest sections while attempting to machine it!)

Not sure what you're referring to regarding the required advanced computing capabilities, but note that the main reason for using equilateral triangles for the mesh (true "isogrid") is that it simplifies the design calculations a whole lot by making the properties of the whole lattice isotropic. A "waffle grid" (rectangular lattice) is much easier to manufacture but much harder to design. "advanced computing" actually enables more intricate and tailored lattice patterns than these two simple examples, so weird lattices have become a hot topic recently.

Regardless of the pattern, if someone is aiming to optimize weight, strength, vibration etc, it will require computing power, which was not available at competitive cost, say, 50 years ago. The sizzling hot generative design patterns that are filling the newsfeed of anyone interested in this subject are a whole different question. But yes, the simplier lattices do not require ludicrous investments form R&D departments nowadays, hence they are more and more common these days.

@Jpe61 Stress analysis has been "done" long ago using scaled models, manufactured from materials that are easier to work with. As long as the model is kept in the linear elastic region, and as long as there's no localized material anisotropy, then whatever you measure on the model applies (with proper scaling) to the full size structure. Some machined designs of this sort required dozens (sometimes >100) manually machined models as the optimization progressed. In some cases the models were machined wax, then investment cast, instrumented, loaded, measured, rinse and repeat.

Yes. Antoni Gaudí modelled the structure of  Sagrada Familia with strings and bags of sand almost 150 years ago. All kinds of modelling has been done for eons, modern computing has taken the optimization of structures to a whole new level.


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Is a civilian pilot allowed to fly under part 91 rules with Night Vision Goggles?
I asked a related question: How hard is it to fly with night vision goggles?
It appears from the answers so far that there are differing opinions, and I guess it might depend on the quality/age/capability of the equipment.
If someone was flying non-commercially (Part 91), Private Pilot (or higher) and it was VFR flight, if the pilot owned a pair of The ANVIS 9, Aviator Night Vision Goggles, Model M949, for example, could they legally fly around with them in their own airplane at night (and be acting as Pilot-In-Command)?
(Assume that they are a safe, prudent aviator, and took efforts to properly train themselves on the equipment and flew with a safety pilot until they were comfortable in the air with the equipment).
Would it be any different if they were flying IFR (appropriately-rated and on an instrument flight plan)?

Theoretically? Yes. In practice? Maybe.
The FARs provide for civilian operations under NVGs, but the story is a bit more complicated.
NVG operations requirements for pilots are covered in 61.31(k) for obtaining an endorsement and 61.57(f) for currency. Reading over the wording, the FAA appears to take the same general approach that they do with tailwheels: i.e. get trained and endorsed and then stay current.
The problem is with the aircraft rather, as 91.205(h) requires, in addition to day and night VFR equipment, a gyroscopic pitch and bank indicator (not typically an issue), a generator or alternator as per IFR reqs (again, not generally an issue), interior and exterior lighting appropriate for NVG ops (may be an issue), and a radar altimeter (definitely an issue in almost all light, and some not-so-light, aircraft).
Not sure about the goggles themselves, maybe there's a TSO out there giving tech specifications for aviation-specific NVGs?

Flying with NVG is quite different from normal flying.
With NVG on, your field of vision is very limited. It is about 40º wide instead of over 180º. This needs training to get used to.
You get the feeling that with NVG, you can see everything without limits, whereas in reality, the level of detail you see at night with NVG is more close to legal blindness if that was your quality of vision during the day. There are also several phenomenon that if you are not trained to recognise, you might fly into a hill that you didn't think was there, because it was not clearly visible due to a larger hill behind it (lack of contrast between the two objects in the field of view) or an object in the shadow of a hill or mountain. High tension wires, trees that don't have leaves, etc.
When you have two used toilet-rolls available, stick them together to make make-shift binoculars, put them on your head like an NVG (try to block your peripheral vision past the tubes to prevent cheating) and try to walk through familiar territory (like where you live) and see how well you do. Then, if you are bold enough to do that, try to cycle like that. I would not recommend you do this while driving unless it is in an empty parking lot and you are trying to avoid pylons or some other soft non-living obstacle. Once you get the idea, imagine doing that at night while presented with much lower quality imagery.
I have no knowledge of the legality of civilian NVG operations

See also How hard is it to fly with night vision goggles?.


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Perpendicular Distance between PAPI and Aiming point
We are in the middle of a new runway construction. The airport requested the aiming point to follow the PAPI. Should they be aligned head to head, or there can be some meters difference?

Hi shirley, is there a reason not to install them such that they are aligned?

Based on the airport preference, it is required to have the aiming point following the PAPI location. So, should it be head by head or it can be only coincide? @Jpe61

I'm sorry, I do not understand what you mean by head by head / coincide.


text:
Does this Lear 35 have missiles on it?
Janes aircraft recognition guide has a picture of this plane on the Lear 35 page. Does it have missiles on it or is this some special modification?

Further photos on http://www.airliners.net/search/photo.search?regsearch=N83FN have more comments, one of which say "30 year old Lear loaded with ACM pods to simulate fighter/missile attacks on surface ships".

No, it's not a missile, it's a towed target:

Towed targets are used for electronic warfare and gunnery training. The can serve as a radar target or visual target during military exercises.
The 'missile' is on a long cord and can be reeled from the aircraft. The pod that you see here is a MTR-101 reeling system with the capability to drag the target up to 30,000 ft behind the aircraft.
If you are interested in flying these kind of missions, it seems  L3 is looking for a new First Officer for their Learjets.

This raises the further question "Do aircraft towing targets ever get shot down?" or are we just talking about targeting practice as opposed to target practice.

Apparently yes. Googling for the tail number turns up more pictures, as well as comments saying this particular plane is "used to simulate missile attacks".
Here is the owning company, a defense contractor that among other things provides simulated air attacks for training exercises.

uh, no. The word SIMULATED says it all. They're not missiles.


text:
What is this component in an Airbus cockpit?
I have noticed that on a lot (all?) Airbus cockpits, there are these "things" (for lack of a better term) under the panel.
I have highlighted them in this picture of an A340 cockpit:

(source: ourl.ca)
What are they?

Your highlight is that yellow oval below the glass information panel?

@Farhan yes I am ;)

The centre bit is the pull out mechanism for the table, the two bits either side are fold down footrests.

This actually answered my question the best, as it explained the footrests too. Good job. ;)

Footrest? They still have rudder pedals, underneath that, correct?

@RoboKaren Correct. You can see the rudder pedals in the picture above. The foot rests fold down, supported by the two bars. They retract up out of the way when the pilot removes their foot. Boeing aircraft have a less complex solution of two bars attached to the bottom of the main panel above the footwell.

Since Airbus planes use a sidestick instead of the traditional center yoke, they use the space where the yoke would be to include a fold out table instead. You can see a laptop on one here. The A380 version also has a keyboard. This is used to interface with the onboard systems and input information.

Ask an Airbus pilot what their favourite part of the aircraft is and they all say 'the tray table!'

Are you talking about this area?

This is a pull out keyboard, which is part of Electronic flight bag (EFB). It is part of a standard laptop computer. However, its integrated Keyboard Cursor Control Unit (KCCU) can interact with the cockpit screens. The full-size keyboard felt substantial and, in the paperless cockpit, will greatly ease the tasks of data and text entry.

Note that this is A380 cockpit where the EFB is integrated. The earlier designs only have a table for maps/charts/handbooks/checklists in paper or regular notebook form.


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Attack angle in high speed

I found the above animation; the moving slow air (or wing) will hardly to lead to an aerodynamic stall, but the usual case is a plane in the air running much faster than the animation, so does the wing with such attack angle and high speed make an aerodynamic stall?

It is really difficult to understand what you are asking. I think I get it, but it shouldn't be this difficult to figure out or guess what you are after. Please, if English is your second language I would urge you to find a CFI or aviation-smart mentor in your native language who can help further your understanding. Alternately, if they don't have the answers, perhaps someone could help you translate your question into a grammatically correct sentence.

Stalls happen at (or above) the critical AoA regardless of speed.
However, lift varies with AoA and speed, so if you increase the airspeed at the same AoA, the plane will climb.  Assuming you want to maintain altitude, when you increase airspeed, you will also reduce the AoA to compensate.
Stalls almost always happen in the reverse of that scenario: the aircraft is slow, which means the pilot must increase AoA to keep generating enough lift to maintain altitude. But without adding sufficient thrust to compensate, this reduces airspeed and thus lift, so the pilot increases AoA further. This vicious cycle continues until the critical AoA is reached and the wing stalls.


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Where does the altimeter measure from?
During the landing flare, the cockpit can sometimes be twenty/thirty feet or more above the rear landing gear, especially with larger aircraft.

(source: valkaviation.com)
In some aircraft, you can hear the system call out altitude when on final, ie "500... 200... 100... 50... etc".
Where does the altimeter measure altitude from? Does it measure from the bottom of the rear landing gear, or from the cockpit itself? How do pilots know when the rear landing gear will touch down?

There are a few types of altimeters but they all generally do the same thing and can be calibrated to anything if need be. Where they measure from and what they consider 0 are really 2 different questions.
A pressure altimeter reads from the pito-static port (many planes have more than one of these for redundancy) and they are generally located forward on the air frame and often on the side somewhere.

((737 static port) source)
Radar Altimeters will most likely mount to the bottom of the aircraft (since that is the way the radio beam needs to go).

(source)
According to this discussion radar altimeters are on the belly and 0ft is calibrated to mains on the ground, after that altitudes will be read out in negative values.
And from here

The callouts (50, 40,30,20,10 feet, etc) is the function of two radio
  altimeters which provide the height of the main landing gear above the
  runway.

Might be worth mentioning that the pressure altimeter can be calibrated to read either field elevation or zero when the aircraft is on the ground.

@Dave: Did you mean to write "pitot" rather than "pito"?

The altitude callout is usually given using the radio altimeter. The radio altimeter (Tx/Rx) is usually located in the forward fuselage and calibrated to tell the altitude of the main landing gear above the ground.

Location of Boeing 757 radio altimeter; image from flightdatacommunity.com
This patent gives details of a system used for calibration of the radio altimeter. From the document:

To ensure that a radio altimeter indicates zero altitude when the aircraft is on the ground, an on-ground calibration system utilizes a calibration switch to initiate a calibration radio altimeter signal from the radio altimeter's transmitter section which is reflected off of the ground and received by the altimeter's receiver section. The time delay between transmission and reception of the signal is measured and a value representing the delay is stored in the altimeter's microprocessor. This value corresponds to zero altitude and can be used as the relative zero point for calculating altitudes of the aircraft when it is in flight.

This gives the basics of operation of the altimeter. You can set the MLG altitude as the zero point for landing (of course once the aircraft is in hangar and the landing gear is compressed, it will show negative values).


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How would an aircraft visually signal "in distress"?
A ship at sea would fly its flag upside down to signal it's in distress. How does a plane or airliner that has no radio or transponder visually signal that it is in distress?
This question: How do commercial pilots send distress signals? explains how it is done using a radio or transponder, but how could the message "please don't shoot me down" be relayed visually to a fighter pilot?

To fly a flag upside down the aircraft would probably have to fly inverted :D "Look at me I'm in distress, I'm inverted" LOL

A flag upside down? Wouldn't do much good for Romania, France, Belgium and Japan, would it?

It seems like there are multiple questions here. Are you meaning specifically to ask about how to communicate with a fighter pilot who is escorting you if you've lost your radios or are you asking more generally about how to signal distress. It would be better to focus the question on one or the other, as this seems to be causing confusion in the answers.

@Mast That's some very impressive flag knowledge. Username checks out.

I'm trying to imagine a situation in which "please don't shoot me down" would ever be a useful message ;-D

I remember a stolen MIG indicating a desire to land by dropping its landing gear at a South Korean base, but that wasn't distress.

Annex IV of the "International Regulations for Preventing Collisions at Sea" (a.k.a., "COLREGS") lists fifteen different things that the crew of a ship can do to signal a dire emergency.  Flying an up-side down flag is not one of them.

I would presume that every aircraft wants to communicate that particular message.

Once we got into trouble on a French ship, so we flew our flag upside down.  Nobody helped.  Then we flagged British and had another problem.  We were ignored by 11 ships until a boat of British vexillologists stopped to help.   Then we flagged American, ditto.   Everybody stopped!

Presumably, a plane could plummet towards the ground at high speeds to signal distress.

Have you tried pleading and praying? At this point of time, if you do not know how to follow the military aircraft and cannot communicate using radio or you lights a prayer is all I would have left.

For the US, the FAA's Intercept Procedures list several things a pilot can indicate without radios:

Acknowledge instructions: rock wings, flash nav lights
Unable to land at indicated airport: flash landing lights
Cannot comply: switch all lights on and off at regular intervals
In distress: switch all lights on and off at irregular intervals

Are the any other ways if those cannot be performed?

Being creative: If near enough, sheets of paper with one big letter on each? A torch or mirror? Cellphone if low enough (normally a nogo, but if in distress...).

@Peter Yes a satellite phone.

@Muze Handheld radio, mobile phone, gestures, Morse code with a flashlight, writing in large letters on a sheet of paper, and who knows what else. I'm guessing that people will get creative in that situation :-)

I dimly remember that flying in a triangular pattern is supposed to be a distress signal for ATC radar.

@JohnK See this question :-)

Although not a commercial pilot example, during Operation Frequent Wind (US evacuation from Saigon during Vietnam War) "(...)the pilot of an RVNAF Cessna O-1 dropped a note on the deck of the carrier. The note read "Can you move these helicopter to the other side, I can land on your runway, I can fly 1 hour more, we have enough time to move. Please rescue me. Major Buang, Wife and 5 child."

@Peter I'm guessing you're in the UK? The thought of signaling with a torch while flying an airplane sounds like a very bad idea in American English. - haha

For your convenience „flashlight“. I‘m not a native speaker.

@Peter haha - Yeah, I knew what you meant. It just caused a funny mental picture. I suppose flames in the cockpit would be very effective at signalling distress, though.

I seen footage of planes in the 2WW waggling/rocking their wings to mean they're in need of help + don't shoot. In more modern films where a 'goodie' is flying back home in a stolen enemy aircraft - flying on a steady path, and rocking wings has been used to portray the 'don't shoot' - but obviously that's films, so pinch of salt needed.

@Peter "torch" is actually correct in British English; "flashlight" in American English :)

Oh dear.  So the trigger-happy interceptor pilot is supposed to know if the lights are flashing at a "regular" or "irregular" interval?  Maybe the pilot being intercepted should hold up a box of Pepto-Bismol or Immodium.

ICAO describes the following distress signals in Annex 2 (Rules of the Air):

The following signals, used either together or separately, mean that  grave  and  imminent  danger  threatens,  and  immediate assistance  is  requested:
a)  a  signal  made  by  radiotelegraphy  or  by  any  other signalling   method   consisting   of   the   group   SOS (. . . — — — . . .  in  the  Morse  Code);
b)  a radiotelephony distress signal consisting of the spoken word MAYDAY;
c)  a distress message sent via data link which transmits the intent  of  the  word  MAYDAY;
d)  rockets or shells throwing red lights, fired one at a time at short intervals;
e)  a parachute flare showing a red light.

Note that a) mentions "any other signalling method", which could, for instance, be flashing aircraft lights. d) and e) require no radio or transponder.
In addition, the following urgency signals exist:

The   following   signals,   used   either   together   or separately,  mean  that  an  aircraft  wishes  to  give  notice  of  difficulties  which  compel  it  to  land  without  requiring  immediate assistance:
a)  the  repeated  switching  on  and  off  of  the  landing  lights;
b)  the  repeated  switching  on  and  off  of  the  navigation lights  in  such  manner  as  to  be  distinct  from  flashing navigation lights

Are there any signals which could be sent from an aircraft which lost all electrical systems? (I'm assuming most planes don't carry flares or rockets.)

@Skyler In that case, rocking the wings could still be useful on short range.

@Skyler: "...a [SOS] signal made by radiotelegraphy or by any other signalling method..." -- Interceptors will attempt to get a look into the cockpit for an idea of what's going on. A torch or smartphone light will do nicely.

Commercial flights are very regular throughout the world. One sure fire way to signal "don't shoot me I'm in distress" would be to fly low, and slow. a 737 at 5000 AGL would certainly signal something is wrong (most of the time) and would certainly get someone's attention.
Finding an airport and following the failed comms procedures and making a landing there would also be a sure sign that you needed help.
Remember you don't need a transponder and radio to land, and if you're doing something unusual, generally speaking, everyone else gets out of the way. Also, it would depend on the kind of distress your in. With no radio and no transponder, you probably have a pretty big electrical issue and need to land right away. So land.

Your advice goes against the procedure linked to in Pondlife's answer, which says: "Do not adjust your altitude, heading, or airspeed until directed to by the intercepting aircraft."

Those rules are for being intercepted. i.e. a fighter escort. I was speaking to letting others know you're in trouble.

Yeah, I think the question isn't very clear about exactly what it's asking for. This seems to answer the title question, but not the part in the body about, "how could the message "please don't shoot me down" be relayed visually to a fighter pilot?" That part would indeed imply being escorted. Having said that, if your goal is to indicate that you are not a threat and should not be shot down, flying a 737 at 5,000 ft probably isn't the best first move. It will signal distress alright, but not so much the "Please don't shoot me down" part.

I think the downvoted are for suggesting flying a 737 at 5000 ft to get attention. While it would get you attention, I think it’s also a good way of getting shot down.

@reirab Why would you need to signal to your own escorts to not shoot you down? You'd signal that to interceptors if anything.

@Kapten-N: I think the question here is to turn the interceptors (which would have to take into account the possibility of a 9/11 type of situation) into escorts (i.e. pilots who no longer wonder whether they will have to shoot you down, and instead attempt to help, e.g. by relaying morsed light signals to ground).

@DevSolar Yes. That was my point exactly. You're trying to signal interceptors, not escorts.


text:
How much do "Preferred Routes" matter for non-airliners?
So, full disclaimer that this may be a IRL vs. VATSIM thing, but I'm curious about how difficult it is to get cleared for an IFR flight plan that takes a (literally) scenic route?
E.G. I want to go KBOS to KORH. The preferred route for this is direct TEC @10,000. But I'm taking a friend who used to live east of Manchester, NH with me and he wants to see his old house from the air. His old address is roughly in the vicinity of KHRIS, east of KMHT.
My expectation is that if I file, for example, KBOS MANCH KHRIS CLOWW SPENO KORH (basically the first half of KBOS->KMHT, then the KRHIS waypoint to overfly my buddy's old address, and then the preferred route KMHT (who I'm in the vicinity of) to KORH (my actual final destination) clearance delivery will clear me IFR to Worcester airport, direct, wiping out the route I desire to fly.
And if that's what's gonna happen, how do I communicate to clearance delivery that I'm not drunk, the flight plan I filed is the one I actually want to fly?

Why are you trying to do this IFR?

Why not do your sightseeing VFR and then pick up IFR in the air to your destination afterwards?

@Jim Because the example is an excuse-plot to illustrate my question. What I'm actually trying to do is understand more of the how/why of how strictly IFR flight plans and preferred routes are enforced by ATC.

Flight plan processing, and the route you are assigned, is done by the overlying ARTCC computer (ERAM). ERAM looks at your aircraft type, requested altitude, requested route, and destination to arrive at the actual route assigned. In congested airspace like KBOS, it is unlikely that ERAM will let you fly anything other than the pref route. You can make the request with CD, but it would require a bunch of coordination with other controllers. The reason those adapted routes exist is because over time, facilities agreed that was the best way to flow traffic that fit given parameters.

I appreciate the history here, ultimately "why?" is a question my brain insists on answering before it will agree to retain any information for me.  I'm curious if you can expand this with anything about further reading, and/or under what conditions and for what purposes unorthodox routing might be accepted?

There's a saying in ATC, "You can do anything with coordination." CD just has to be willing to coordinate with all affected facilities (and get their approval) to put an aircraft where it normally wouldn't be if they had followed ERAM adaptation. Think of asking the State Patrol to block one lane of head-on traffic so you can drive the wrong way down the Interstate. Can it be done? Yes. Is it likely to occur?

The short answer (when it comes to real filing) is it depends.
You are free to file what ever you like (direct route included). Ultimately its up to ATC/FAA to assign your route and they are free to give you back what ever they want. You can file direct, and get cleared direct, you can file direct and get mouth full of a route on read back, it depends on the day, the route, the weather, other traffic, etc. But ultimately you are expected to fly the route assigned.
Generally people fly planes to get somewhere faster than they otherwise could so its not uncommon for GA pilots to frequently file direct and simply take the route you are then assigned if its not direct (this is what I do usually). Since 100% of my filing is electronic these days (via Foreflight) the read backs are not nearly as daunting since I almost always have the route in front of me (updates are sent back to foreflight after filing if you dont get what you filed)
Some thing you can try to get the route approved:
Tell clearance delivery it is an IFR training/practice flight and you would like to fly the filed course. There is nothing preventing you from asking, but dont expect approval.
If you are trying to over fly a specific point, fly from your home airport to another airport that will likely dump you on an IFR route that overflies your point of choice.

"If you are trying to over fly a specific point, fly from your home airport to another airport that will likely dump you on an IFR route that overflies your point of choice."  I don't suppose there's any option to request routing to a new destination once airborne, even if you've got cell service and can electronically re-file in mid-air?

Sure there's an option, it sounds like this: "Approach, Skychicken 3AB, request."

@WilliamWalkerIII as noted you can ask for amendments in the air, you can also always pick up an IFR clearance from flight service if you are VFR and airborne already.

I want to know what airline's call-sign is "Skychicken"! That sounds like a fun airline to fly on ! ;)

@FreeMan Nok Air maybe. :)

Wouldn't it be easier to just file VFR? I'm also going to assume that if you want to overfly your buddy's old house, you'll need to be VFR to see it.
If you actually fly that route, MEA from MANCH to KHRIS is 2300, so you'd have to be at least 3000`.
Maybe it would be easier to go VFR, contact MHT Tower, let them know you're going to be maneuvering in the vicinity, then head on out on the rest of your journey. You'd be able to descend with less hassle so your friend can see their house.

The example motivation is really just an excuse plot to illustrate my question, I grabbed KHRIS randomly off a chart of MHT's neighborhood.
Yes, filing for VFR would be easier, but I'm trying to wrap my brain around how strict preferred routing is for IFR flight plans, and this seemed like as reasonable a hypo to stress-test it as any.

I've done corporate customer acceptance flights and our flight plans were typically big lazy circles, following a circular chain of VORs usually, coming back to the departure airport (the completion center).  You can also request maneuvering area clearances within specific dimensions once airborne. Sometimes we would need to do a bit of maneuvering (the customer acceptance crew on board checking out the interior would want to make sure nothing is loose during G loading). We'd request an altitude block, and permission to maneuver between fix X and fix Y, within so many miles of the airway.

@JohnK What would that look like on the flight plan? Or is that something you'd do live?

Just departure airport, waypoint A, B, C etc, arrival airport (being the same).   You would file it like that.  While flying it, if you want to change it, say to make a short cut home, you just request the change on the go.  You'd call them up with a clearance change request and say ,"We'd like to proceed direct X (bypassing waypoint W) then continue from X to destination as cleared." or something like that.


text:
what is Limit load factor structural and aerodynamic envelope at structural design gross weight and maximum gross weight?
As an aerospace engineer, as soon as I saw the words "structural and aerodynamic envelope" the first thing that popped into my mind that is it V-n diagram? But when I searched about it in the internet I found so many different answers. Now I am confused. I need this answer for a competition documentation part. They want this envelope for a VTOL aircraft.

What have you found for example? And what's the exact question from the competition documentation? Do you have a link to it?

i found velocity vs altitude graphs of some helicopters, but i need this for a VTOL aircraft
link- https://vtol.org/files/dmfile/rfp_sikorskyhvstol_40thsdc_2022-23_final.pdf
2.4.9 (1)

What a very interesting RFP they have this year! Anyway I also don't get what they're asking about here: there's a way to ask question to the VFS?


text:
Why is the lift coefficient not 0 at the initial time t* = 0?
In an experiment on pitching airfoil. Experimental results are given by the following graph, the curves are the evolutions of the lift coefficient.
Why is the lift coefficient not zero at the initial time t* = 0?

At t* = 0 then the angle of attack is 0 degrees. As we know at AoA = 0, CL must be 0 right?
Link to the paper Unsteady lift on a high-amplitude pitching aerofoil

I suppose it depends on the definition of AoA - if zero is the point at which Cl is zero then it does seem to be paradoxical, but if it’s based on a line between the most widely-spaced extremes of the wing section then some lift might be expected at this angle.

Let me know if this answer of mine is of any help. Basically when the airfoil oscillate above a certain speed then $C_l$ is not only proportional to $\alpha$ but also to the speed and the acceleration by which $\alpha$ changes

What is "k" please?

@RobertDiGiovanni: k is the reduced frequency. You find the definition in my answer I linked here above.

@sophit thanks.  Not ASE related, but I wonder if marine life benefits from any of this as they oscillate (swim).  Fish seem incredibly good at holding their place in a current with minimal motion.

@RobertDiGiovanni: fishes remain aloft using buoyancy, they are more like Zeppelins than airplane, the less they move the better they hold their place.

@sophit  not buoyancy, propulsion.  Holding place in a current.  The reverse stroke can recover some energy of (water moved by the) 1st stroke.

"As we know at AoA = 0, CL must be 0 right?". Only for symmetrical airfoils. The angle where CL is null is the angle of zero lift. See How do you calculate the lift coefficient of an airfoil at zero angle of attack?